In CT, an external x-ray source is caused to be passed around the patient. Detectors around the patient then respond to x-ray transmission through the patient to produce an image of an area of study. Unlike PET, which is an emission tomography technique because it rely on detecting radiation emitted from the patient, CT is a transmission tomography technique which utilizes only a radiation source external to the patient.
PET is a branch of nuclear medicine in which a positron-emitting radiopharmaceutical such as 18F-fluorodeoxyglucose (FDG) is introduced into the body of a patient. Using compounds such as 11C-labeled glucose, 18F-labeled glucose, 13N-labeled ammonia and 15O-labeled water, PET can be used to study such physiological phenomena as blood flow, tissue viability, and in vivo brain neuron activity. Positrons emitted by these neutron deficient compounds interact with free electrons in the body area of interest, resulting in the annihilation of the positron. This annihilation yields the simultaneous emission of a pair of photons approximately 180 degrees apart. The radiation resulting from annihilation is detected by a PET tomograph. More specifically, each of a plurality of positrons reacts with an electron in what is known as a positron annihilation event, thereby generating a coincident pair of 511 keV gamma rays which travel in opposite directions along a line of response (LOR). After acquiring these annihilation “event pairs” for a period of time, the isotope distribution in a cross section of the body can be reconstructed.
A PET scanner is used to detect the positron annihilation events and generate an image of at least portions of the patient from a plurality of detected events. The PET scanner may comprise a plurality of radiation-sensitive PET detectors arrayed about an examination region through which a patient is conveyed. The PET detectors typically comprise crystals and photomultiplier tubes (PMTs). The detector crystals, also referred to as scintillators, convert the energy of a gamma ray into a flash of light that is sensed by the detector PMT. In coincidence mode a gamma ray pair detected within a coincidence time by a pair of PET detectors is recorded by the PET scanner as an annihilation event. During a patient scan hundreds of million of events are typically detected and recorded. Due to the approximate 180 degree angle of departure from the annihilation site, the location of the two detectors registering the event define the LOR passing through the location of the annihilation. Detection of the LORs is performed by a coincidence detection scheme. A valid event line is registered if both photons of an annihilation are detected within a coincidence window of time. Coincidence detection methods ensure that an event line is histogrammed only if both photons originate from the same positron annihilation. The observed events are typically sorted and organized with respect to each of a plurality of projection rays. By histogramming these lines of response, a “sinogram” is produced that may be used by, for example, a process to produce a three dimensional image of the activity. All events occurring along each projection ray may be organized into one bin of a three-dimensional sinogram array. The array may be stored in a computer-readable memory media. The sinogram data is then processed to reconstruct an image of the scanned volume.
Prior to image reconstruction, efficiency normalization techniques are used to correct the sinogram data for non-uniform PET detector responses due to, for example, PET scanner geometry, detector crystal non-uniformity, and gain variation in detector PMT's. Efficiency normalization may be regarded as being quality check procedures for PET or PET/CT and may be done in several different ways. One method may include placing a uniform phantom cylinder in the field of view of the PET and scanning this cylinder for a certain time (for example 200 million counts) to acquire a uniform phantom sinogram. Thereafter, such a method may compute crystal efficiency arrays from the uniform phantom sinogram and from another standard quality check sinogram, a normalization sinogram. The system quality may then be determined based on a statistical valuation, such as the chi-square value, between these two crystal efficiency arrays. By comparing if any errors between the computed crystal efficiency of the uniform phantom sinogram is within a selected statistical (for example a chi-square value) range of the crystal efficiency of the normalization sinogram it may be determined if the quality of the PET scanner is within an acceptable range.
For example, scanning of a known uniform phantom object on a regular basis and store its sinogram may result in a standard sinogram that is stored for a specific PET scanner. When a quality check is subsequently performed, for example every morning, the same uniform phantom object is placed in the field of view of the PET. By comparing if any errors of a computed crystal efficiency of the newly obtained sinogram is within a selected statistical (for example a chi-square value) range of the crystal efficiency of the stored standard sinogram it may be determined if the quality of the PET scanner is within an acceptable range.
A chi-square test is any statistical hypothesis test in which the test statistic has a chi-square distribution when the null hypothesis is true, or any statistical hypothesis test in which the probability distribution of the test statistic (assuming the null hypothesis is true) can be made to approximate a chi-square distribution as closely as desired by making the sample size large enough. Specifically, a chi-square test for independence evaluates statistically significant differences between proportions for two or more groups in a data set.
It takes the same requirement to acquire the uniform phantom sinogram as it takes to acquire the normalization sinogram. In certain situations the normalization sinogram is needed frequently, for example daily in a mobile environment. Daily quality checks must re-acquire the uniform phantom sinogram and compare it with the normalization sinogram acquired minutes ago. This creates extra data acquisition time and is not an efficient usage of data.
In view of the quality check procedures for PET or PET/CT, but also for other tomography procedures, it is desirable to provide a quality check procedure that is short in time. Additionally or alternatively, the quality check procedure should make efficient use of existing data and processing capacity.
It is desirable to provide a quality check procedure that will result in an improve quality of the image reconstruction procedure and the resulting image. Besides the continuous quest for improved image quality, it may be desirable to have an efficient and/or sensitive medical device performing PET, PET/CT, SPECT or SPECT/CT. This would allow for a reduction in time for taking images, an improved quality of the images, and/or a reduction of exposure of a subject to the image apparatus.
Additionally, it is desirable to avoid cumbersome and time consuming arrangements or methods for checking quality, in an economic and technical perspective.